With increasingly stringent emissions and efficiency regulations there is an increasing push toward the use of alternative fuels in internal combustion engines. In the context of the present disclosure, the term “internal combustion engine” comprises applied-ignition Otto engines especially. In the case of Otto engines, for example, not only gasoline as the traditional Otto engine fuel but also, for example, liquefied gas (LPG—Liquefied Petroleum Gas), a propane/butane mixture, which is also referred to as autogas, or natural gas (CNG—Compressed Natural Gas), primarily methane, are being used as fuels. Hydrogen (H2), ethanol or fuel mixtures consisting of gasoline and ethanol are further examples of alternative fuels.
In the case of the internal combustion engine which forms the subject matter of the present disclosure, not only a liquid fuel, e.g. the traditional Otto engine fuel, but also a gaseous fuel, e.g. natural gas, is used as a fuel for operating the internal combustion engine, wherein, in the context of the present disclosure, the term “gaseous fuel” is used when the fuel is in the gaseous phase under ambient conditions.
Since different fuels have different physical and chemical properties, the internal combustion engine may be designed specifically for the fuel used. In this context, adaptation of the operating parameters of the internal combustion engine, e.g. of the ignition point and of the injection point, may be required. The timings, the boost pressure, the cooling water temperature, the injection duration, the charge air quantity, and also design parameters, e.g. the compression ratio, can be, and frequently are, designed for operation with a particular fuel. The fuel used also affects the design configuration of the fuel supply system of the internal combustion engine, in particular the injection system used to introduce the fuel.
A fundamental distinction may be drawn here between two concepts of fuel injection, namely intake pipe injection and direct injection. In the case of intake pipe injection, the Otto engine operates with a substantially homogeneous fuel/air mixture, which is prepared by external mixture formation in a process in which fuel is introduced into the induced charge air in the at least one intake line of the intake system. The load is adjusted by modifying the fill of the cylinder, e.g. by means of quantity control, the usual method used with Otto engines, generally by means of a throttle valve provided in the intake line. By adjusting the throttle valve, the pressure of the induced air downstream of the throttle valve can be reduced to a greater or lesser extent. The further the throttle valve is closed, e.g. the more the intake line is blocked, the higher is the pressure loss in the induced air across the throttle valve and the lower is the pressure of the induced air downstream of the throttle valve before entry to the cylinder. With a constant combustion chamber volume, it is possible in this way, using the pressure of the induced air, to adjust the air mass, e.g. the quantity. This method of load control proves particularly disadvantageous in the part-load range since low loads require severe throttling and a large pressure reduction in the induced air. This results in high pumping losses. In order to reduce these throttling losses, e.g. these pumping losses, various concepts have been developed.
Injecting fuel directly into the combustion chamber of the at least one cylinder is regarded as a suitable measure for dethrottling the internal combustion engine and making a noticeable reduction in fuel consumption, even in the case of Otto engines. By means of direct injection, it is possible to achieve a stratified combustion chamber charge and hence, within certain limits, quality control. Moreover, exploiting the enthalpy of vaporization of the fuel introduced directly into the combustion chamber results in an internal cylinder cooling effect which allows a further increase in efficiency by raising the compression ratio.
The disadvantage with direct injection is that there is relatively little time available for injection of the fuel, mixture preparation in the combustion chamber, namely preparation of the fuel, possibly by vaporization, mixing of the charge air and the fuel and ignition of the prepared mixture. Otto-cycle processes which employ direct injection are therefore significantly more sensitive to changes and deviations in mixture formation, especially during injection and during ignition, than Otto-cycle processes which employ intake pipe injection.
The prior art includes internal combustion engines in which intake pipe injection for introducing gaseous fuel into the intake system and direct injection for introducing liquid fuel into the at least one cylinder are provided. Such an injection concept for an internal combustion engine which can be operated alternately by means of liquid or gaseous fuel has a large number of disadvantages, which will be briefly explored below.
Different sorts of fuel have different levels of resistance to knock, these being indicated by the octane ratings RON and MON. The compression ratio & of the Otto engine, e.g. the compression ratio & of the at least one cylinder, may therefore be designed for the fuel with the lower knock resistance. In general, the liquid fuel is the fuel with the lower knock resistance. A relatively high compression ratio which allows problem-free operation of the internal combustion engine with the gaseous fuel can then lead to knocking in the case of operation with liquid fuel. Since knocking or spontaneous ignition has to be reliably avoided, the internal combustion engine may be designed for the fuel with the lower knock resistance.
It should be noted in this context that the efficiency η of the internal combustion engine correlates more or less with the compression ratio ε, e.g. efficiency η is greater at a higher compression ratio ε and lower at a lower compression ratio ε. The fact that the internal combustion engine has to be provided with a lower compression ratio ε to match the fuel with the lower knock resistance has the effect that the efficiency that could theoretically be obtained with the use of the gaseous fuel cannot be achieved, e.g. the actual efficiency potential of the gaseous fuel is not exhausted.
Introducing the gaseous fuel into the intake system of the internal combustion engine by means of intake pipe injection has the effect that the high-pressure gas in the intake system expands before and during its introduction into the at least one cylinder in the course of exhaust and refill. In the case of internal combustion engines pressure-charged by means of exhaust gas turbocharging, in particular, this has the disadvantageous effect that the turbocharger may perform additional volumetric work in order to recompress the gas that has expanded in the intake system. In order to introduce the same mass of mixture into the cylinder, a higher boost pressure is therefore also required.
Since the internal combustion engine is preferably operated with gaseous fuel or is to be operated with gaseous fuel whenever, as soon as and for as long as gaseous fuel is available, the second operating mode is the preferred operating mode, and hence the operating mode which is to be used more frequently in normal operation of the internal combustion engine.
This has the effect that the injection device, e.g. an injection nozzle, arranged in the combustion chamber of the at least one cylinder in order to introduce the liquid fuel is unused for relatively long phases. Without continuous use of the injection nozzle for the purpose of fuel injection, the lack of the cooling which is generally brought about by the fuel introduced may lead to high temperatures in areas of the injection nozzle which face or are adjacent to the combustion chamber. The high temperatures can lead to thermal overloading of the nozzle and also to coking, with even very small quantities of fuel left behind on the injection device during injection burning incompletely if there is a deficiency of oxygen.
Deposits of coking residues form on the injection device. On the one hand, these coking residues can modify the geometry of the injection device in a disadvantageous way and affect or hinder the formation of the injection jet, thereby noticeably disrupting mixture preparation. On the other hand, fuel injected in the first operating mode is deposited in the porous coking residues and then often burns incompletely if there is a deficiency of oxygen, forming soot which, in turn, contributes to an increase in particulate emissions.
Moreover, coking residues may break away, owing, for example, to mechanical stress due to a pressure wave propagating in the combustion chamber or the action of the injection jet. The residues broken away in this way can lead to damage in the exhaust system and can, for example, impair the ability to function of exhaust gas aftertreatment systems provided in the exhaust system.
Owing to the high temperatures, coke deposits may also form within the nozzle, and these can not only affect or hinder the formation of the injection jet and disrupt mixture preparation but, what is more, can jeopardize the ability to function of the nozzle overall.
The effects and actions described above have the effect that the internal combustion engine may generally be operated in the first, liquid fuel operating mode in order to counteract damage to the nozzle or problems due to coking. This is not conducive to operation of the internal combustion engine in the second, gaseous fuel operating mode as frequently and for as long as possible.
The inventors recognized the above described disadvantages and disclose a system and method for prevention of coking residues as described below. According to the disclosure, in contrast to the prior art, the gaseous fuel is injected directly into the cylinder when the internal combustion engine is in the second operating mode, whereas the liquid fuel is introduced into the intake system by means of intake pipe injection if the internal combustion engine is being operated in the first operating mode.
The present disclosure describes a system for an engine comprising a direct injection nozzle for injecting gaseous fuel into a cylinder of an engine in a second operating mode; an intake injection nozzle for injecting liquid fuel into an intake port of the engine in a first operating mode; and a valve gear suitable to adjust timing of opening and closing of an inlet valve. Preferential injection of a gaseous fuel such as compressed natural gas directly into the cylinder increases efficiency and allows for reduced heat exposure to the lesser used liquid gas injectors mounted in the intake port, reducing coking of these injectors.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Further, the inventors herein have recognized the disadvantages noted herein, and do not admit them as known.